1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly to an electrophotographic image forming apparatus and an electrophotographic image forming method using a photoreceptor having a photosensitive layer including a charge generation layer and a charge transport layer as an electrostatic latent image bearer (hereinafter referred to as an “electrophotographic photoreceptor” and a “photoconductive insulator” as well).
2. Discussion of the Background
Recently, development of information processing systems utilizing electrophotography is remarkable. In particular, optical printers in which information converted to digital signals is recorded using light have been dramatically improved in print qualities and reliability. This digital recording technique is applied not only to printers but also to copiers, and so-called digital copiers have been developed and used. Copiers utilizing both the conventional analogue recording technique and this digital recording technique have various information processing functions, and therefore it is expected that demand for such copiers will be escalating. In addition, with popularization and improvement of personal computers, the performance of digital color printers which can produce documents including color images has been rapidly improved.
Conventionally analogue image forming methods have been used for electrophotographic image formation. Analogue image forming methods typically use a posi-posi developing method. However, currently digital image forming methods are typically used and almost all of these image forming apparatuses use a nega-posi developing method. This is because almost all images to be produced by these image forming apparatuses are character images, which typically have a relatively low image area proportion of from 5 to 10%.
Conventional analogue image forming methods typically use a posi-posi developing method in which a charged photoreceptor is exposed to a light image which is prepared by irradiating an original image, and a non-lighted portion, which is an image portion and has a relatively high potential, is developed with a toner, resulting in formation of a toner image. In contrast, digital image forming methods typically use a nega-posi developing method in which a charged photoreceptor is exposed to a light image, and a lighted portion, which is an image portion and has a relatively low potential, is reversely developed with a toner, resulting in formation of a toner image. The image forming methods using a nega-posi developing method have an advantage in that the output time of a light source (such as laser diodes) of a light irradiating device can be dramatically reduced (to about one-tenth).
In the nega-posi developing method, a non-image portion (i.e., a non-lighted portion) of a photoreceptor has a high potential even after a developing process. Therefore, the photoreceptor is subjected to a discharge process after the transfer process. Specific examples of the discharging methods include optical discharging methods in which light irradiates the photoreceptor to cancel the residual charge by the photo-carriers generated by light irradiation; mechanical discharging methods in which an electroconductive member such as brushes is contacted with the photoreceptor to leak the residual charge; electrical discharging methods in which a reverse bias is applied to the photoreceptor to cancel the residual charge; etc.
Recently, electrophotographic image forming apparatuses can produce high definition images and color images. Therefore, information (i.e., original images) input to such image forming apparatuses to be produced is slightly changed from character images to photograph images, color pictures and graphs, etc. When such images are produced, a problem in that the resultant images have a ghost image of a previously formed image occurs unless the charge remaining on the photoreceptor is discharged. A ghost image is typically formed as follows. When a residual charge is insufficiently discharged, the photoreceptor has an uneven potential after being charged. When light irradiates such a photoreceptor to form an image (particularly a half tone image), the resultant electrostatic latent image has an uneven potential. When such a latent image is developed, a ghost image of the image formed in the last image forming operation is formed in the resultant toner image.
There are two causes for formation of a ghost image. One of the causes is that since image formation is performed at a high speed, there is a case where the capacity of the charger used is insufficient for evenly charging a photoreceptor having a residual charge. In this case, a ghost image is caused. The other of the causes is that a charging roller is used as a charger of an image forming apparatus to miniaturize the image forming apparatus (particularly tandem type image forming apparatus). Charging rollers, which cause discharging between the surface thereof and the surface of a photoreceptor, cause a ghost image relatively easily compared with conventional charging device such as corotrons and scorotrons.
In any event it is important to maintain uniformity of the residual potential of a photoreceptor (i.e., the potential of a photoreceptor just before charging). Therefore, in order to produce high quality images, the discharging process is very important now.
Among the various discharging methods mentioned above, the methods except for the optical discharging methods have the following drawbacks. Specifically, since the discharging methods using a brush or the like contacts the member with a photoreceptor, the photoreceptor and the member are easily abraded, and thereby the lives of the photoreceptor and the member are shortened. In addition, the methods cause a problem in that when the surface of the photoreceptor or the member is contaminated with a toner or the like, the discharging effect is deteriorated. Further, the methods cannot perform discharging at a high speed, and therefore the methods are not suitable for high-speed image forming apparatuses.
The electrical discharging methods applying a reverse bias to a photoreceptor have a drawback in that when the bias is too low, even discharging cannot be performed, and when the bias is too high, the photoreceptor is reversely charged (i.e., the photoreceptor has positive charges). Since general photoreceptors can transport only positive charges, positive charges formed on the photoreceptors cannot be cancelled. When the thus positively charged photoreceptor is negatively charged in the following charging process for forming an image, the photoreceptor is charged so as to have a predetermined negative potential after the positive charges thereon are cancelled by the negative charging. Therefore, the negatively charging tends to be insufficiently performed, resulting in formation of an uneven residual potential on the photoreceptor. In addition, when positive charges are formed, traps are formed in the photosensitive layer, and thereby a residual potential is easily formed on the photoreceptor. In this case, the life of the photoreceptor is shortened.
Thus, the optical discharging methods are preferable for electrophotographic image forming methods and apparatuses at the present time. As mentioned above, images to be produced by an image forming apparatus typically have an image area proportion of 10% at the highest. Therefore, 90% or more of the surface of a photoreceptor is discharged (i.e., photo-carriers are generated in 90% or more of the photosensitive layer to discharge the residual charges) when the nega-posi developing method is used whereas 10% or less of the surface of a photoreceptor is discharged when conventional image forming methods using a posi-posi developing method are used. Therefore, the discharging process has been hardly studied until now.
Japanese Laid-Open Patent Publication No. 60-88981 or 60-88982 discloses an image forming apparatus which uses a photoreceptor including an inorganic photosensitive material (such as selenium alloys and amorphous silicon) and which uses a discharging device emitting light having a relatively short wavelength to reduce fatigue of the photoreceptor caused by the light irradiation and charging. However, the photoreceptor disclosed therein is an inorganic photoreceptor and therefore the technique cannot be applied to organic photoreceptors as it is. This is because the photo-carrier generation mechanism of inorganic photoreceptors is different from that of organic photoreceptors. In addition, the image forming apparatus uses a posi-posi developing method, and therefore the technique cannot be used for nega-posi developing methods as it is because the influence of the discharging on residual charges in nega-posi developing methods is different from that in posi-posi developing methods. Further, as a result of the present inventor's experiment, it is found that the discharging device, which emits light including a component with a wavelength of not less than 500 nm, cannot produce good discharging effects.
Japanese Laid-Open Patent Publication No. 61-36784 discloses a discharging technique in that light used for discharging a photoreceptor including a photosensitive material whose photosensitivity is improved by a dye has a wavelength which is substantially identical to the specific wavelength at which the non-sensitized photosensitive material has a photosensitivity (i.e., which is not the wavelength at which the dye has absorption). For example, when a photoreceptor using polyvinyl carbazole which has absorption in the ultraviolet region and whose sensitivity to visible light is improved by adding a dye (which has absorption in the visible region) thereto is used, a discharging device emitting light having a wavelength in the ultraviolet region is used. In this case, when discharging is performed using ultraviolet light, the photo-carrier generation efficiency is low and thereby discharging cannot be efficiently performed. In addition, the photosensitive material (i.e., polyvinyl carbazole) is easily deteriorated by the ultraviolet light. Therefore, the technique is not effective. Further, this technique is used for posi-posi developing methods, and therefore the technique cannot be effectively used for nega-posi developing methods.
Japanese Laid-Open Patent Publication No. 62-38491 discloses a discharging technique in that light having a relatively short wavelength range irradiates a photoreceptor having a photosensitivity in a relatively long wavelength region and having lower or little photosensitivity in the relative short wavelength range to prevent fatigue of the photoreceptor caused by the light irradiation. However, when the technique is used for high speed image forming apparatuses, the discharging effect is poor, resulting in formation of a ghost image. Namely, the technique cannot be applied to current image forming apparatuses. In addition, Japanese Laid-Open Patent Publication No. 62-38491 does not specify the wavelength range of the discharging light.
Japanese Laid-Open Patent Publications Nos. 01-217490 and 01-274186 disclose discharging techniques in that light with a wavelength of not greater than 620 nm irradiates a positive-chargeable photoreceptor having a layered photosensitive layer in which a charge generation layer is formed on a charge transport layer. The light used for discharging includes light with a wavelength of not less than 500 nm. As a result of the present inventor's experiment using these techniques, the residual charge decreasing effect is insufficient.
Japanese Laid-Open Patent Publication No. 04-174489 discloses a discharging technique in that two kinds of light emitting diodes irradiate a photoreceptor to prevent increase of residual potential of the photoreceptor under high temperature and high humidity conditions. The light used for discharging includes light with a wavelength of not less than 500 nm. As a result of the present inventor's experiment using this technique, the residual charge decreasing effect is insufficient.
Japanese patent No. 3460285 discloses a discharging technique of using discharging light having light intensity, which exceeds the half value of the maximum absorption peak of the photosensitive layer of the photoreceptor used, at a wavelength within the wavelength range between the lower and upper half values of the maximum absorption peak, wherein the photosensitive layer is a single-layered photosensitive layer including an organic pigment. In general, organic pigments used as photosensitive materials have absorption in the visible region, and thereby light with a wavelength of not less than 500 nm has to be used for the discharging light. As a result of the present inventor's experiment using the technique, the residual charge decreasing effect is insufficient.
Japanese Laid-Open Patent Publication No. 2002-287382 discloses a discharging technique in that discharging is performed using light to which the photoreceptor used has a higher sensitivity than that to the image writing light. It is described therein that by using this technique, the residual potential can be reduced and thereby formation of a ghost image can be prevented. In Japanese Laid-Open Patent Publication No. 2002-287382, the wavelength of the discharging light changes depending on the photosensitive material used for the photoreceptor and therefore the wavelength is not specified therein. In general, organic pigments have absorption in the visible region. Therefore there is a case where light with a wavelength not less than 500 nm is used for discharging. In this case, the residual charge decreasing effect is insufficient.
Japanese Laid-Open Patent Publication No. 2005-31110 discloses a discharging technique in that light, against which the photoreceptor used has relatively low absorption, irradiates the photoreceptor to discharge residual charge thereon, wherein the photoreceptor has a single-layered photosensitive layer in which a charge generation material is dispersed. This light irradiation is performed to remove charges generated within the photosensitive layer. Specifically, in a case of single-layered photosensitive layer, a charge generation material is uniformly dispersed in the entire photosensitive layer. Imagewise light, against which the photosensitive layer has relatively high absorption, is absorbed by the surface portion of the photosensitive layer, and therefore photo-carriers are formed in the surface portion. However, the charges formed in the inner portions of the photosensitive layer far from the surface portion remain therein while being trapped. The thus trapped charges cannot be cancelled by the discharging. In attempting to solve the problem, light which has such a relatively long wavelength as to be able to enter into the bottom portions of the layer is used as discharging light to generate photo-carriers therein, and cancel the trapped charges with the photo-carriers. However, in general the photosensitive layer of a photoreceptor having a layered photosensitive layer is relatively thin compared to single-layered photosensitive layers. In addition, the image writing light is absorbed by the layered photosensitive layers at a rate of not greater than 90% (i.e., 10% or more of the image writing light passes through the photosensitive layers. Therefore, charge generation is performed in the entire photosensitive layers unlike the single-layered photosensitive layers even when the wavelength of the image writing light is changed. Therefore, the effect described in Japanese Laid-Open Patent Publication No. 2005-31110 is not produced for photoreceptors having a layered photosensitive layer.
Japanese Laid-Open Patent Publication No. 2004-45996 discloses a discharging technique of using discharging light having a wavelength corresponding to the soret band of a phthalocyanine compound used for the photosensitive layer. It is described therein to use a fluorescent lamp as a discharging light source.
It is also described in Japanese Laid-Open Patent Publication No. 2004-45997 to use a fluorescent lamp as a discharging light source for a photoreceptor including a phthalocyanine compound as a photosensitive material.
FIG. 1 illustrates the emission spectrum of a fluorescent lamp. The spectrum includes several emission lines, and a high emission line is observed at a wavelength of from 500 to 650 nm. Since the quantities of components of the fluorescent light are proportional to the areas of the peaks, the components of the light having a wavelength of from 500 to 650 nm mainly irradiate the photoreceptor. In Japanese Laid-Open Patent Publications Nos. 2004-45996 and 2004-45997, discharging using a LED emitting red light with a wavelength of 680 nm is compared with discharging using a fluorescent lamp. Since a fluorescent lamp mainly irradiates light having a wavelength of from 500 to 650 nm, discharging using light having a wavelength of from 500 to 650 nm is compared with discharging using light with a wavelength of 680 nm. Although each light includes a component corresponding to the soret band of a phthalocyanine compound, the light quantity of the component is small. Therefore, the discharging method hardly produces a good effect.
In addition, image forming apparatuses are required to produce high quality color images and to have high durability. In order to produce high quality images in digital image forming apparatuses, one of the key points is to form a clear and small one-dot electrostatic latent image and the other of the key points is to prevent formation of abnormal images. In addition, it is important to prolong the life of the photoreceptors used for the image forming apparatuses. In order to develop the key technologies, it is important to reduce fatigue of a photoreceptor, specifically it is important to prevent increase of residual potential of lighted portions of a photoreceptor.
In order to prevent increase of residual potential of lighted portions of a photoreceptor, the materials used for the photoreceptor and the formulation of the layers of the photoreceptors have been studied. However, the fatigue of photoreceptor largely depends not only on the formulation of the layers of photoreceptors but also on the image forming conditions of image forming apparatuses. Therefore, it is the conventional way of researchers and developers that materials and formulations are studied to develop a photoreceptor suitable for the target image forming apparatus. In other words, it has not been performed to study fatigue of photoreceptors from the viewpoint of image forming conditions.
Because of these reasons, a need exists for an image forming apparatus and method which can produce high quality images while preventing increase of residual potential of the photoreceptor used for the apparatus even after long repeated use.